48 A YEAR IN SCIENCE 



increased when it becomes steam. All of us know of 

 the force exerted by steam. This enormous force is 

 utilized in all the numerous kinds of steam engines. 



The return from the gaseous to the liquid state, or 

 from the liquid to the solid state, is always accom- 

 panied by a considerable decrease in volume. 



Molecular changes resulting from change in state. 

 We already know that if the temperature of a solid is 

 increased, its volume is also increased. We also know 

 that this is because the molecules of the solid have 

 been driven farther apart. By continuous heating, 

 the molecules of a solid may be driven, or forced, far 

 enough apart actually to change the form of the sub- 

 stance from that of a solid to that of a liquid. Simi- 

 larly, when the molecules of a liquid are forced far 

 enough apart, the liquid becomes a gas. 



Anything which interferes with the free movement 

 of the molecules will increase the amount of heat neces- 

 sary to change, for example, a liquid to a gas. Conse- 

 quently, if the air pressure on a liquid is increased, 

 its boiling point is raised. The reverse is also true, 

 that if the pressure upon a liquid is reduced, the mole- 

 cules can move apart more readily, and consequently, 

 less heat is necessary to convert the liquid into a gas. 



We thus see that by decreasing the pressure upon 

 solids or liquids, less heat is necessary to change them 

 into gases. 



It is also true that to change a gas to a liquid, or a 

 liquid to a solid, the molecules must be brought closer 



